1. Field of the Invention
The invention relates to a substrate structure having ball pads, particularly to a substrate structure having non-solder mask design ball pads.
2. Description of the Related Art
The conventional substrate used for BGA IC package has two types: one is solder mask design (SMD) substrate, and the other is non-solder mask design (N-SMD) substrate. For the SMD substrate, there is no gap between the solder pads and the solder mask of the substrate, so that holes will exist in the solder ball because the solder material on a printed circuit board comprising volatile flux when the BGA IC package substrate is welded with the printed circuit board. Therefore, the reliability of solder ball welding may decrease.
FIG. 1 and FIG. 2 show a conventional substrate structure 1 having N-SMD ball pads for BGA IC package. The substrate structure 1 comprises a substrate 10, a trace layer 11 and a solder mask 12. The substrate 10 has a first surface 101, and at least one ball pad 102 is disposed on the first surface 101. The trace layer 11 is disposed on the first surface 101 and is electrically connected to the ball pad 102. The solder mask 12 is used to cover part of the trace layer 11. The solder mask 12 has an opening 121 formed on the corresponding position of the ball pad 102. The shapes of the opening 121 and the ball pad 102 are round, and the size of the opening 121 is larger than that of the ball pad 102. And, part of the trace 11 is exposed between the opening 121 and the ball pad 102.
Referring to FIG. 3, in order to improve the problem of holes in the solder ball of the SMD substrate, the size of the opening 121 of the substrate structure 1 having N-SMD ball pads is larger than that of the ball pad 102, so that the air in the solder ball 14 can be exhausted through the space between the opening 121 and the ball pad 102 when the substrate structure 1 is welded with a printed circuit board 13. Therefore, the problem of holes in the solder ball 14 can be avoided, and the reliability of solder ball welding can be improved. However, the solder mask 12 only covers part of the trace 11, so that the solder ball 15 will be dragged by the exposed part of the trace 11, and non-alignment and displacement of the solder ball 15 therefore occur when proceeding with a subsequent reflowing step for mounting solder ball 15 on the ball pad 102, as shown in FIG. 4.
Consequently, there is an existing need for providing a substrate structure having non-solder mask design ball pads to solve the above-mentioned problems.